Posted
by
timothy
on Saturday December 17, 2011 @10:40AM
from the minor-suckage dept.

smitty777 writes "Adding to the recent black hole discoveries of gas clouds and a quasar accretion disc, Forbes is reporting on a recent discovery by NASA's Rossi X-Ray Timing Explorer (RXTE) on the smallest known black hole. From the article: 'If the astronomers' calculations are correct, this black hole is located about 16,000 to 56,000 light years away from Earth (a more precise distance hasn't yet been determined). The black hole itself is only about three times the mass of the Sun, which means that the original star was just barely big enough to form a black hole.'"

WHAT THE FUCK! Is this for real? Maybe I'm just imagining this, but this article looks like, dare I say it, it may just be "news for nerds!" After months of articles about various shitty Apple devices, copyright, American politics, and shitty Apple devices again, it's stunning to finally see an article about a scientific discovery again.

What's worse, the daydreaming adolescents, or those that act like they have something better to do but keep droning on about how bad it is. If you don't like it so much, why don't you actually contribute something to the discussion?

There have been a bunch of claims of black holes roughly in the range 3-4x solar masses, some subsequently revised upwards (this one [wikipedia.org] made some news in 2008, and there are some other candidates as well). The "normal" range for stellar black holes is roughly 3-30x solar masses, according to current understanding.

Anyone have a link to a good explanation of the current estimated values for a minimum? My understanding is that there isn't really a theoretical physical minimum (black holes can exist at any size), but that there's a mass level beneath which astrophysicists consider it very unlikely that conditions would have really existed to produce a black hole through stellar collapse of a star. But I can't seem to find a solid estimate of what that number is, just these sorts of indirect references to 3x being "close" to the minimum (looking at Google Books, I find an old textbook that also mentions 3.2x as "just above" the theoretical minimum, but doesn't elaborate).

Replying to myself: it appears that the minimum is related to the Tolman–Oppenheimer–Volkoff limit [wikipedia.org] for the maximum mass of a neutron star, which isn't known to great accuracy. Wikipedia cites a 1996 journal article with an estimate of "approximately 1.5 to 3.0 solar masses".

Here [wikipedia.org] it's claimed that the limit for a white dwarf collapsing to a neutron star is about 1.4 solar masses. Some statistics of how common black holes are relative to neutron stars could probably narrow down that 1.5 to 3.0 maximum for a neutron star quite a bit. At the lower end neutron stars should be fairly rare, shouldn't they?

There is, I believe, another limiting factor in Hawking radiation. Because the area of the horizon relative to the mass of the hole is larger the smaller the mass, a too small hole will "evaporate" (for lack of a better term) in an accelerating process, before it winks out with a "tzing!"

There is no fixed "limit" for this, nor do I believe one would make much sense, as it would also depend on how well fed the black hole is. And unless in our back yard, or rather, on our doorstep, we can only observe the b

A black hole that would grow by absorbing radiation instead of shrinking at the ~4K average temperature of the universe can be around the mass of the Moon. Such a small black hole cannot form just by gravitational collapse but can go on indefinitely unless the universe cools down.

The question here was about the minimum mass a star can have to become a black hole instead of remaining a neutron star - or maybe something more exotic but still fighting gravity. The Tolman–Oppenheimer–Volkoff equation gives an estimate but since noone managed to observe exactly what happens with matter at the densities found in a neutron star there are still a lot of assumptions.

Evaporation is an issue for very small black holes, and usually important to question of primordial black holes that have been around since when the early universe could have created them without requiring a star collapse. While a kilogram sized black hole would evaporate on the order of femtoseconds, and even a million kilogram one would be gone in a minute, the evaporation time goes with mass cubed. So by the time you get to a single solar mass, it is 10^67 years to evaporate, as it is radiating at abou

Replying to myself: it appears that the minimum is related to the Tolman–Oppenheimer–Volkoff limit [wikipedia.org] for the maximum mass of a neutron star, which isn't known to great accuracy. Wikipedia cites a 1996 journal article with an estimate of "approximately 1.5 to 3.0 solar masses".

There is a recent observation of a neutron star with 2.0 solar mass (+/- a small uncertainty).

I may be wrong as I'm not a physicist, but as I understand it that chandrasekhar limit applies only if a black hole is formed through a star collapsing. If a black hole is formed by some other means then its mass could be something entirely different, including the minimum limit for forming one. As wikipedia so helpfully lists there are two known ways for a black hole to form: gravitational collapse and high-energy collisions. There could be some as-of-yet-unknown means, too.

The more precise answer to the OP's question thus would seem to be: the Planck mass [wikipedia.org]

As wikipedia so helpfully lists there are two known ways for a black hole to form: gravitational collapse and high-energy collisions. There could be some as-of-yet-unknown means, too.

Black hole forming from high-energy collisions is poorly understood, as on that scale both quantum and gravitational effects play a role, and there is no solid quantum gravitational theory. Theoretically, there could be other ways of black hole formation, such as ones that formed in the Big Bang, but those are also areas that we know little about. So yes, in some exotic theories small black holes could exist, but those theories are all unconfirmed.

I am a physicist, although not an astronomer. Indeed, microscopic black holes (less than the earth mass) are speculated
to exist. They're called primordial black holes [wikipedia.org] and must be created in the early universe.

"I may be wrong as I'm not a physicist" Uh, you could be a physicist and still be wrong.

Yeah, especially considering that we actually don't have correct laws of physics to work with. We have vague approximations that do us just fine at the macro level for some things, but when it comes to what's really happening, we're no better off than the blokes who coined the term "Atom" and then found it actually IS divisible after all.

That's definitely one hard limit, but I believe the open question is whether there are other minimum bounds for a black hole formed through stellar collapse, which may be higher than the Chandrasekhar limit. The Chandrasekhar limit is the maximum size of a white dwarf, but there may be other ways of preventing collapse into a black hole at higher masses; for example, the TOV limit that governs neutron star formation.

Link desired? I'm not a physicist, but I remember "Schwartzchild radius"from high school, and, as per usual, Wikipedia fills in the blanks comingup with closed-form solutions for stellar black holes near 3 solar masses,to wit:

Astronomical errata aren't exactly what people turn to Forbes for. I keep waiting for the punch line... Is it a bank? A country that threatens the stability of the Euro? A cell phone manufacturer that bet its future on WP7?

Maybe the punch line is that it's too small to have been formed by stellar collapse, and therefore is evidence of a former intelligent alien species who have built an LHC and created a black hole that way, which then ate their star.:-)

black hole "radius", or the radius of its event horizon, is related to its mass.for a non rotating black hole: R=2G*M/c^2G being newton constant, M the black hole's mass, and c speed of lightif you take M to be three times the mass of the sun, you'll get R=8860 meters

This is the probably the last big science release for Rossi X-ray Timing Explorer (RXTE). RXTE will be turned off at the end of this year and disabled. This is mostly because of NASA budget reductions and reduced prioritization by peer review committees, but also because of its aging on-board systems. The systems were designed for a two year life-time but lasted for more than sixteen! The satellite will probably remain in orbit for quite a few more years - passive - before re-entering the atmosphere. The scientific community will lose the only working X-ray observatory that can measure the fast heartbeats of black holes and neutron star systems, and do complicated monitoring observations.

RXTE has done a lot of great science in the past 16 years, some of it featured [slashdot.org] here [slashdot.org] on Slashdot [slashdot.org]. The legacy will live on though, since the data archive will remain publicly available. Sometimes great science can come from the archive as well!

Here [arxiv.org] is the scientific paper. It makes no claim whatsoever about the mass of IGR J17091-3624. On p. 6, they say:

Figure 5 implies that if IGR J17091-3624 emits at Eddington, then either it harbors the lowest mass black hole
known today (< 3Msolar for distances lower than 17 kpc),
or, it is very distant. Such a large distance, together with
its b ~2.2deg Galactic latitude, would imply a significant,
but not necessarily implausible, altitude above the disk

Here [nasa.gov] is the NASA press release summarizing the paper for people who aren't scientists. It quotes the lead author as saying:

Just as the heart rate of a mouse is faster than an elephant's, the heartbeat signals from these black holes scale according to their masses

The Forbes article morphs this into "NASA Satellite May Have Found The Smallest Known Black Hole," and says, "An international team of astronomers utilizing NASA's Rossi X-Ray Timing Explorer (RXTE), believe that they've identified a candidate for the smallest known black hole[...]"

The slashdot summary says:

The black hole itself is only about three times the mass of the Sun[...]

This is completely incorrect. It's a candidate for a very low mass black hole. What that means is that they're suggesting that astronomers do follow-up observations on this object and actually determine its mass, which may be unusually low.

It is of very great interest to relativists and astronomers to find the smallest black holes. There is a limit called the Tolman-Oppenheimer-Volkoff limit on the largest mass that a neutron star can have. There are big theoretical uncertainties in this number, but it is probably around three solar masses. However, we don't know for sure whether anything too massive to be a stable neutron star necessarily becomes a black hole. There have been all kinds of goofy objects hypothesized by theorists that might be intermediate between neutron stars and black holes, including black stars, gravastars, fuzzballs, quark stars, boson stars, and electroweak stars. Observing a low-mass black hole narrows the gap in mass between the heaviest stable neutron star and the lightest black hole, leaving less wiggle room to believe in these exotic objects.